According to CaptainJean-Luc Picard in the role of Dixon Hill, the Second World War, although disastrous, did end with the United States taking its place as a dominant world power, culturally influencing the second half of the 20th century. Additionally, the war was a catalyst of technological advancements. The developments in rocketry and fission resonate on into the 24th century. (TNG: "Manhunt")

"Today the guns are silent. A great tragedy has ended. We have known the bitterness of defeat and the exultation of triumph, and from both we have learned there can be no turning back. We must go forward to preserve in peace what we've won in war."

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In one alternate timeline, the Na'kuhl apparently helped the Germans win the war (although whoever assassinated Vladimir Lenin in 1916 probably helped more, by removing the threat of the Communists to the Nazis). The Nazis subsequently sent troops and conquered Britain, the northeastern United States, a part of Russia extending past Moscow, and possibly parts of eastern Canada. The Nazis took over New York City after the US military ordered a hasty retreat to the Midwest, causing refugees to flood over the Brooklyn Bridge. (The Germans subsequently rolled over Flatbush Avenue in Brooklyn and took over Washington, DC, and possibly parts of Virginia.)

However, this was described as Germany "stretching too far, too quickly". Resistance fighters constantly attacked German supply lines and the frontlines were too thin. An American counterattack could – and was later trying to – break through the frontlines, when Captain Archer and the Enterprise-NX showed up in New York City. The Germans also faced problems in their stronghold in Africa, while Russian forces attempted to retake Moscow.

An alternate timeline also exists in which Germany won the war due to the delayed entry into the war by the United States. This was caused by the prevention of Edith Keeler's death by DoctorLeonard McCoy, who had traveled to the past from the 23rd century. Keeler was an anti-war activist, whose activism had persuaded the United States to stay out of the war long enough to ensure a Nazi victory. This errant timeline was repaired by James T. Kirk and Spock.

In the altered reality, Edith eventually met with President Roosevelt, effectively organizing pacifist movements to ensure that the United States did not join the Allied Powers and contribute to the escalation of war. Without the US's help fighting them, Germany completed its heavy-water molecular experiments, created an atomic bomb first in the new history, and used their V-2 rockets to launch them, effectively capturing the world. This timeline was averted when McCoy was followed through the Guardian of Forever time-portal, by Kirk and Spock, who prevented him from interfering in her death, restoring the 'correct' version of history. (TOS: "The City on the Edge of Forever")

In TOS: "Bread and Circuses", Spock states that eleven million people died in World War II. This, however, severely contradicts current data, which places the death toll anywhere from a low of twenty million (if only military is counted) to a high of over seventy million (if military and civilians are counted). Most counts settle on around fifty million or so.

Spock may have been referring solely to civilian casualties as a result of "slavery" and "despotism" rather than the total war itself. 11 million is a number frequently given for civilian deaths solely from Nazi policies. This might explain the six million figure he gave for World War I, too, as that is about an average (slightly on the low side) estimate of civilian deaths out of a World War I total of about sixteen million.

According to the novella Age of the Empress, the Second World War also occurred in the mirror universe, with Japan as one as its participants. One of the results appears to have been a lessening of the power of Japan's emperor, as the Kyoto Imperial Palace was turned into a tourist attraction.